Optical communication technologies using light of a visible light range have recently been attracting much interest. Along with the rapid proliferation of lighting devices using light emitting devices such as Light Emitting Diodes (LEDs) or the like, techniques for realizing more convenient, higher-rate data communication using infrastructures including indoor and outdoor lightings are under development.
Although LEDs are most promising for light emission in high-speed optical data transmission in terms of influence on the human body and medical equipment, other semiconductor light emitting devices are also available, inclusive of Laser Diodes (LDs) or Super Luminescent Diodes (SLDs) that offer a faster response performance. Also, data rate is dependent on the response speed of a light emitting device in optical communication. In this context, a technique for transmitting a large amount of data stably in one signal of light emission of a light emitting device is needed.
In relation to the above problem, for example, Japanese Laid-Open Patent Application Publication No. 2004-147063 (Patent Document 1) discloses a structure for adjusting a control voltage to change the intensity of light emitted from a light source according to a frequency-multiplexed signal. With the use of this structure, it is possible to change the light intensity of a light source or transmit multiplexed data under on/off control. Compared to non-multiplexing, a higher data rate can be achieved. Japanese Laid-Open Patent Application Publication No. 2006-325085 (Patent Document 2) discloses a structure for transmitting different data by controlling light emission of a plurality of light sources having different spectrums. To be more specific, Patent Document 2 discloses a structure for transmitting data by mapping digital values (1, 0) to phase changes at a transition timing of a Pulse Width Modulation (PWM) signal.